1. Field of Endeavor
The present invention relates to a method of reducing emissions and flashback in a sequential combustion gas turbine, and to a combustor for such a gas turbine.
2. Brief Description of the Related Art
A gas turbine with sequential combustion is known to be able to improve the efficiency and to reduce the emissions of a gas turbine. This can be achieved one way by increasing the turbine inlet temperature. In sequential combustion gas turbines, engine fuel is combusted in a first combustor and the hot combustion gases are passed through a first turbine and subsequently supplied to a second combustor, known as an SEV combustor, into which fuel is introduced through a lance projecting into the combustor. The combustion of the hot gases is completed in the SEV combustor and the combustion gases are subsequently supplied to a second turbine.
SEV combustors were originally designed for natural gas and oil operation. The prior art SEV combustor design poses challenges in terms of both durability and higher chances of auto ignition (premature ignition) or flash back occurrence when operated on syngas or fuels with high H2 content. A flashback event is a premature and unwanted re-light of the premixing zone, which produces an order of magnitude increase in NOx emissions and causes significant damage to the burner parts.
New combustor designs for use with syngas or hydrogen rich fuels, such as MBTU, involve redesigning the fuel injector systems to mitigate risks of flash back. The new injector designs take into account the very high reactivity of H2 containing fuels, however the walls of prior art SEV combustors are effusion air cooled and the carrier air convectively cools the lance system. This cooling has proved to be insufficient, leading to durability problems.
Experience has shown that there is an additional need for the SEV combustor to be redesigned to cope with the radically different combustion properties of hydrogen rich fuels such as MBTU, which have lower ignition delay time, higher adiabatic flame temperatures, and higher flame speeds. A higher flow rate of the fuel is also required due to the lower density of hydrogen rich fuels compared to traditional fuels such as natural gas. The application of existing designs to such harsh fuels can result in high emissions and safety issues. To improve the SEV combustor design it has also been suggested to increase dilution of the gas flow or improve the form of the SEV combustor which requires extensive development and validation efforts which are expensive to implement.